In the process of establishing an oil or gas well, the well is typically provided with an arrangement for selectively excluding fluid communication with certain zones in the formation traversed by the well to avoid communication with undesirable fluids. A typical method of controlling the zones with which the well is in fluid communication is by running well casing down into the well and then sealing the annulus between the exterior of the casing and the walls of the wellbore with cement. Thereafter, the well casing and cement may be perforated at preselected locations by a perforating device or the like to establish a plurality of fluid flow paths between the pipe and the product bearing zones in the formation. Unfortunately, the process of perforating through the casing and then though the layer of cement dissipates a substantial portion of the energy from the perforating device and the formation receives only a minor portion of the perfcrating energy.
Conventional perforating systems can be fixed inadvertently in that they are usually armed when extended into a wellbore and stray electrical signal or charge can cause premature firing which is a potential safety and operation problem.
Conventional perforating systems are quite expensive especially in terms of the rig time necessary to run a perforating gun into a well. This is particularly true for tubing conveyed systems where pipe is made up to run a perforating gun into a high angled hole for example.
Additionally, when completing high angled or horizontal borehole sections in a well, whether cased or in open holes, it is often a problem getting perforating apparatus into the high angled section because the gravity factor may not be sufficient to assist in running the equipment and friction between the equipment and borehole walls or pipe further hinders such operations. For this same reason it is difficult in many cases to run casing into such a well, and when casing is installed, the typical bow spring centralizers which are used are ineffective and it is therefore difficult to center the casing in the borehole in order to effectively cement the casing.
Accordingly, it is an object of the present invention to provide a method and apparatus for opening a flow path between the casing string and the formation in a wellbore which overcomes or avoids the above noted limitations and disadvantages of the prior art.
It is a further object of the present invention to provide a method and apparatus for perforating a wellbore wherein a casing string is centered in the wellbore to provide for effective cementing of the casing even when installed in a high angle borehole, and also wherein perforating charges are directed into the formation without penetrating the casing pipe and additionally wherein the casing pipe is in an underbalanced pressure condition.
It is yet another object of the present invention to provide a safe and less expensive method and apparatus for establishing a fluid communication path between the interior of casing pipe set in a borehole and an earth formation by using explosives to selectively opening a plurality of flow path devices mounted in the casing wall.